The invention resides in a method for operating an ultrasound oscillator with a constant oscillation amplitude, including an ultrasound oscillation system which comprises an ultrasound oscillator and components forming therewith an oscillation circuit and to which an excitation voltage is applied to generate an excitation current, wherein the excitation voltage has a frequency outside a resonance frequency of the oscillation system and the size of the excitation current is adjustable by changing the frequency of the excitation voltage.
Furthermore, the invention resides in a circuit arrangement for operating an ultrasound oscillator in accordance with the method described above, including an amplifier with an input and an output which provides the excitation voltage and the excitation current for the ultrasound oscillator system and an oscillator whose frequency is adjustable at a control input and whose output is connected to the input of the amplifier and, furthermore, a current sensor for determining the excitation current.
Such a method and such a circuit arrangement are known in the state of the art and are used for example in the ultrasound welding apparatus manufactured and sold by the assignee of the present application.
For ultrasound welding apparatus, it is important that the energy input into a respective workpiece is constant. It is therefore necessary that the oscillation amplitude of the ultrasound oscillator is constant since the energy input into a respective workpiece depends on the oscillation amplitude of the welding head. This means that the energy input into a workpiece depends on the oscillation amplitude of the ultrasound oscillator. Since the oscillation amplitude of the ultrasound oscillator depends on the excitation current of the ultrasound oscillation system including the ultrasound oscillator and the components forming together therewith an oscillation circuit, the oscillation amplitude of the ultrasound oscillator is maintained constant by keeping the excitation current of the ultrasound oscillation system constant.
In order to facilitate the control of the excitation current, the ultrasound oscillation system is not operated at its series resonance but generally with a frequency which is between the series resonance and the parallel resonance of the ultrasound oscillation system. Since, by changing the frequency at which the ultrasound oscillation system is operated, the impedance of the ultrasound oscillator can be changed, the current flow through the ultrasound oscillation system can be changed by changing the operating frequency of the ultrasound oscillation system.
If during the operation of the ultrasound oscillation system the current flow through the ultrasound oscillation system is changed, for example as a result of outer influences, the frequency of the excitation voltage applied to the ultrasound oscillation system is changed until the excitation current of the ultrasound oscillation system has again reached the previous value.
It has been found however that, in spite of a high quality control of the excitation current to a constant value, the oscillation amplitude of the ultrasound oscillator will deviate.
It is the object of the present invention to provide a method for operating an ultrasound oscillator and respectively, a circuit arrangement by which the constancy of the oscillation amplitude of the ultrasound oscillator is improved.